This invention relates to a method and a device for measuring light transmittance. More particularly, this invention relates to the qualitative or quantitative analysis of body fluids by measuring light transmitted from a coating, such as a reagent coated on the light emitting surface of a lamp. The invention is useful for detecting the presence of one or more constituents of a liquid test sample.
The spectral characteristics of light reflected and transmitted by a specimen are related to the chemical and physical characteristics of the specimen. With the advent of instrumental analysis, absorption of light by a specimen under test has become a widely used basis for biochemical assay procedures. For example, a reagent test strip used for qualitative and quantitative analysis can be contacted for a prescribed period of time with a body fluid, such as blood or urine. Light is projected onto the strip and light reflected by the strip is photoelectrically measured. The reflectance spectrum of the test strip will vary depending upon the concentration of the target substance in the body fluid being examined. Thus, by measuring the reflected light and computing the resulting reflectance, the desired analysis can be made by comparison with a standard based on a correlation of reflectance to concentration.
A reagent test strip is typically a test paper treated with various chemical reagents. Since the surface of the test strip does not have a uniform topography, it is difficult to make accurate and reproducible reflectance measurements because the result may depend upon where the measurement is made on the strip or on measurement geometry. In addition, some illumination from the light source is dissipated or lost. Thus, in order to obtain reflected energy of a desired magnitude, the magnitude of the illumination must be correspondingly increased to compensate for the light dissipated in the device. This can require a larger light source and the generation of heat in larger amounts and can lead to increased power consumption.
It has also been found with reagent test strips that the reflectance measurement is sensitive to the orientation of the strip in the test strip holder. For example, slight twisting or inclination of the specimen affects the magnitude of the reflectance measurement. In addition, it has been found that the reflectance measurement can be affected by the distance between the test strip and the light source.
Thus, there exists a need in the art for a method and a device for making rapid, accurate and reproducible optical measurements on a specimen under test. The device should employ direct illumination of the specimen. The device should minimize the amount of illumination dissipated or lost between the light source and the specimen in order to minimize power consumption. The device should provide optical measurements that are insensitive to subtle changes in the orientation of the specimen in the specimen holder and not susceptible to variations in the distance between the light source and the specimen.